The invention relates to fastening systems for securing rails to concrete railroad ties. In particular, the invention relates to fastening systems having two-piece insulator spacers. The invention also relates to the two-piece insulator spacers. The invention further relates to methods of securing a rail to a concrete railroad tie using such two-piece insulator spacer and to methods of retrofitting a railroad system having a rail insulated from a shoulder insert mounted in a concrete railroad tie using such a two-piece insulator spacer.
Concrete railroad ties have been used in modern railroads for many years. One of the various fastening systems that have been developed for securing rails to concrete railroad ties is shown in FIG. 1. At each rail seat area 2 where a rail 4 is to be fastened to concrete railroad tie 6, cast iron shoulder inserts 8, 10 are provided opposing each other on the field and gauge sides of the rail seat area 2, respectively. Each of the shoulder inserts 8, 10 is permanently mounted within the concrete railroad tie 6 at a position directly adjacent to the rail seat area 2. The rail 4 is mounted between the two shoulder inserts 8, 10 and upon an elastomeric tie pad 12 that spans the rail seat area 2 between the two shoulder inserts 8, 10. An insulator spacer 14 is placed adjacent to and abutting the base or toe 16 of rail 4 between rail 4 and each shoulder insert 8, 10. Each insulator spacer 14 has an inner surface that is adapted to conform to the shape of the vertical and sloping lateral faces of rail base 16. A retaining clip 18, that is attached to a shoulder insert 8, 10 by way of being inserted through a longitudinal receiving hole 20 in a shoulder insert 8,10, presses upon the outer surface 22 of the corresponding insulator spacer 14 to rigidly secure rail 4 to concrete railroad tie 6.
In this system, the tie pad 12 and the insulator spacers 14 act to electrically insulate the rail 4 from its companion rail 4 and from the ground. Such electrical insulation is necessary to permit the rails 4 to be used to conduct electrical signals for monitoring and controlling the progress of the trains that run upon them.
However, electrical insulation is not the only important property that an insulator spacer 14 must possess. The passage of a train upon the rails 4 subjects the rails 4 to complex patterns of horizontal and vertical forces and vibrations. These forces are transmitted from the rails 4 to the fastening systems which retain the rails 4 to the railroad ties. These forces are particularly high on curved portions of the track where the laterally-directed compressive force on a shoulder insert 8, 10 may exceed 28,000 pounds (124,550 N). Because the insulator spacers 14 are sandwiched between the rails 4 and the shoulder inserts 8, 10, these forces subject the insulator spacers 14 to high compressive loads. To combat these loads, insulator spacers 14 have been made of a monolithic, durable insulating material having high compressive strength, such as 6-6 nylon. However, in service, the repeated exposure of the insulator spacers 14 to high compressive loads causes the insulator spacers 14 to deteriorate over time by way of crushing and abrasion. This deterioration occurs mainly in the portion of the insulator spacer 14 that is compressed between the shoulder insert 8, 10 and the vertical or post face 17 of the rail base 16, a portion that is referred to as the post. As the deterioration progresses, the rail 4 becomes able to move, thus causing wear and fatigue on the fastening system components and the concrete railroad tie 6 and compromising the safety of train travel upon the rail 4. Thus, the deterioration makes it necessary to spend time and money to inspect the insulator spacers 14 for wear and to remove and replace worn insulator spacers 14.
It is to be understood that what is being referred to herein by the term insulator spacer is also referred to by those skilled in the art by the simple generic term insulator. However, the term insulator spacer is more descriptive as it brings to mind both the mechanical and electrical functions of the component.
These deterioration problems in their invention which is described in Pilesi et al, U.S. Pat. No. 6,343,748, which is incorporated herein by reference as if set forth herein in its entirety. That invention relates to improved insulator spacers and fastening systems and methods utilizing those insulator spacers. Each of those insulator spacers has one or more composite inserts positioned in its post so that the shoulder insert and the rail each contact the composite insert. Each such composite insert comprises a high compressive strength, electrically insulating material sandwiched between tough outer layers to provide electrical insulation between the rail and the shoulder insert. By locating one or more such composite inserts in the conventional durable, high compressive strength insulating material, e.g., 6-6 nylon, of the insulator spacer""s post, the design of the inventors"" prior invention placed wear resistant, durably tough material in contact with the adjacent surfaces of the rail and the shoulder insert thereby enhancing the mechanical lifetime of the improved insulator spacer of which it is a part.
In addition to possessing good electrical insulation and resistance against deterioration due to crushing and abrasion, an insulator spacer also needs to have the ability to flex appropriately with the applied loads it encounters in service. Two main flexural components may be identified: (1) a linearly-directed horizontal component; and (2) a rotationally-directed vertical component. These are depicted in FIG. 2B respectively by vector arrows X and Y. The linearly-directed horizontal component X compresses the post area of the insulator spacer 24 between the post areas of the rail 4 and shoulder insert 8, 10 causing the post area of the insulator spacer 14 to flex compressively along the direction of the horizontal component X. The rotationally-directed vertical component Y, which is produced by the vertical distortion of the base or toe 16 of rail 4, causes the toe 23 of the insulator spacer to flex upward and the body of the insulator spacer 24 to attempt to rotate around its axis Z. In a conventional, monolithic insulator spacer, the response of the insulator spacer to the vertical component Y conflicts with the compressive flex of the post area of insulator spacer caused by the horizontal component X and results in detrimental stresses in the conventional insulator spacer.
An insulator spacer""s ability to flex is to a significant degree governed by the elastic modulus of its material of construction. The elastic modulus equals the quotient of the applied stress divided by the resulting elastic strain. The elastic modulus is a measure of a material""s stiffness such that the higher the elastic modulus of the material, the stiffer the material is. The optimum elastic modulus value for an insulator spacer depends on the application in which it is used. In some applications, the elastic modulus value that best accommodates the horizontal component X may be different from that which best accommodates the vertical component Y. However, the monolithic structure of conventional insulator spacers militates against, if not completely precludes, optimizing the elastic modulus for both the horizontal component X and the vertical component Y.
The present invention overcomes the problems associated with prior art systems and improves upon the inventors"" own prior invention. The present invention provides an improved fastening system for securing a rail to a concrete railroad tie that employs an insulator spacer comprising a two-piece design consisting of an upper member and a post member. The upper member is pressed upon and thereby fixed in place by the retaining clip that is inserted into a shoulder insert to secure the rail to the concrete tie. The post member comprises substantially all of the post area of the two-piece insulator spacer, although it may extend beyond the post area.
The post member nests into a cavity in the upper member so as to be constrained thereby from migrating laterally and vertically during service. Preferably the post member nests loosely within the cavity, because a loose fit between the two pieces of the two-piece insulator spacer permits independent flexure of the upper member and the post member in response to the loads encountered during service and eliminates the internal stresses caused by the interplay of the linearly-directed horizontal component X and the rotationally-directed vertical component Y of flex described above. Furthermore, because the horizontal component X is primarily associated with the post member and the vertical component Y is primarily associated with the upper member, the elastic moduli for the two-piece insulator spacer can be optimized for both components by constructing the post member to have an elastic modulus optimized for the horizontal component X and the upper member to have an elastic modulus optimized for the vertical component Y.
The post member consists of a composite material comprising a high compressive strength, electrically insulating material sandwiched between tough outer layers to provide electrical insulation between the rail and the shoulder insert. The composite material is designed to place wear resistant, durably tough material in contact with the adjacent surfaces of the rail and the shoulder insert thereby enhancing the mechanical lifetime of the two-piece insulator spacer of which it is a part. The composite material makes the post member sufficiently electrically insulating so as to operably electrically isolate the rail the insulator spacer is in contact with from the shoulder insert with which the insulator spacer is also in contact. By comprising the entire post area of the insulator spacer, the composite material that makes up the post member carries the entire lateral load that the post area is subjected to in service, thereby maximizing the insulator spacer""s resistance to lateral load-induced deterioration.
Morever, the elastic modulus of the post member can be tailored by the selection of the materials comprising the composite material as well as by varying the relative thicknesses of those materials. Thus, for example, where a particular type of steel is selected for the outer layers and a mica-filled phenolic plastic is chosen for the electrically insulating material, the elastic modulus of a composite material of a given overall thickness can be set at any point selected within a significantly wide range by controlling the relative thicknesses of the steel and the mica-filled phenolic plastic. Likewise, if the thickness of each of the layers is selected, then an aim elastic modulus for the composite material can be obtained by properly choosing the layer materials according to their respective elastic moduli.
Thus, described is a fastening system for securing a rail to a concrete railroad tie wherein the concrete railroad tie has a rail seat area on which the rail rests. The fastening system comprises a shoulder insert mounted in the concrete railroad tie adjacent to the rail seat area, a two-piece insulator spacer inserted between the shoulder insert and the rail, and a retaining clip attached to the shoulder insert. The composite material making up the post member of the two-piece insulator spacer is positioned so that it contacts both the shoulder insert and the rail.
Also described is a two-piece insulator spacer having an upper member and also having a post member which consists of a composite material. If desired, nubs or adhesive may be provided for retaining the post member in a cavity of the upper member during the handling of the insulator spacer with the nubs or adhesive becoming inoperative during service of the insulator space; this arrangement allows the insulator spacer to be handled as a single unit prior to service while providing the advantages of independent flexure of the two pieces during service.
Also described is a method of securing a rail to a concrete railroad tie. This method comprises the step of inserting an insulator spacer between a rail and a shoulder insert which is mounted in a concrete railroad tie. The insulator spacer used in this method is of a two-piece construction having a post member consisting of a composite material so that the shoulder insert and the rail each contact the composite material.
Also described is a method of retrofitting a railroad system that has a rail insulated by means of an existing insulator spacer from a shoulder insert which is mounted in a concrete railroad tie. This method comprises the steps of first removing the existing insulator spacer and then inserting between the rail and the shoulder insert a two-piece insulator spacer having a post member consisting of a composite material so that the shoulder insert and the rail each contact the composite material.